This fellowship will support the study of the metabolic control of hepatitis B virus (HBV) biosynthesis. The long-term objectives are to understand the in vivo mechanisms regulating HBV transcription and how alterations in viral RNA synthesis modulate HBV replication. It is proposed to determine the role of the nuclear hormone receptor farnesoid X receptor (FXR) in the modulation of HBV biosynthesis, and the importance of the corepressor, small heterodimer partner (SHP) in regulating FXR mediated HBV transcription and replication under normal and altered physiological conditions. Previous experiments suggested that bile acids should inhibit HBV biosynthesis by activating SHP expression because SHP negatively regulates HBV biosynthesis in vitro. However, our laboratory found that bile acids failed to inhibit HBV replication in HepG2 cells and HBV transgenic mice. Subsequently, FXR was examined because FXR is the major bile acid receptor and FXR activates SHP. Retinoid X receptor (RXR) plus FXR was shown to support replication in nonhepatoma cells. Thus, these experiments suggest the possibility that bile acids fail to inhibit HBV biosynthesis because RXR/FXR directly activates HBV transcription and replication. Therefore, the mechanism of action of RXR/FXR will be characterized to determine its recognition sequence within the HBV genome and its relative responsiveness to various coactivators and corepressors, with a particular focus on the importance of SHP. This functional mapping of the RXR/FXR recognition element in the nucleocapsid promoter will be done by mutational analysis, and to analyze further the relative importance of the recognition elements for RXR/FXR binding, EMSA and DNasel footprinting analysis utilizing nucleocapsid promoter sequences will be performed. In addition, the role of SHP in maintaining tight regulation of HBV transcription and replication will be examined by characterization of SHP null HBV transgenic mice for viral RNA and DNA synthesis under normal physiological conditions and in the presence of cholic acid. Relevance: Hepatitis B places an extensive burden on public health. According to a 2004 report from Centers for Disease Control and Prevention, 2 billion people have hepatitis B serologic markers, 350 million currently have a chronic infection, and there are 0.25-1 million deaths/year in the world from HBV mediated hepatocellular carcinoma. Determining if HBV is a metabolic gene that can be nutritionally regulated, could greatly impact the affliction of hepatitis B on public health.